1. Field of the Invention
The present invention relates to image processing methods and image processing apparatuses for properly processing a character image, gray-scale image and dotted image for high-quality image reproduction, which can be applied to such image processing apparatuses as facsimile machines and image scanners.
2. Description of the Related Art
An image processing apparatus like a facsimile machine which is designed to reproduce an optically read image as a binary image cannot properly reproduce an image by processing a gray-scale image such as photograph through the same image processing method as employed for processing a binary image such as a character image. For proper image reproduction, the character image including characters and diagrammatical drawings is processed by emphasizing the profile of the image (or the boundary between black pixels and white pixels of the image), while the gray-scale image is processed without being subjected to this profile emphasizing process.
However, in the case that an image including both the character image and gray-scale image is read by the facsimile machine, high-quality image reproduction cannot be expected through image processing dedicated for either the character image or gray-scale image. For this reason, the following image processing method has been proposed. When the constituent pixels of an image are successively processed as an object pixel, it is first judged which image area of a character image area and gray-scale image area each object pixel belongs to, and then a proper image processing of the respective image areas can be realized by performing a processing operation for the object pixel in accordance with the judgement result.
An art related to an image processing method for judging which image area of the character image area and gray-scale image area an object pixel belongs to will be described with reference to FIG. 2. An image processing circuit of a facsimile machine processes respective pixels as an object pixel X in the same order as an original-image reading order. When it is judged which image area of the character image area and gray-scale image area the object pixel X belongs to, reference is made to the density values of four pixels A, B, C and D which are located at the four corners of a 3.times.3 pixel matrix having the object pixel X in the center thereof. That is, provided that the characters A, B, C and D also denote the respective multivalued densities of the pixels A, B, C and D, a quadratic differential value S is calculated from the following equation (1): EQU S=.alpha.{(X-A)+(X-D)+(X-B)+(X-C)} (1)
where .alpha. is a given constant.
Then, the quadratic differential value S is compared with a predetermined threshold value, and the image area of the object pixel X is determined based on-the comparison result. This is based on a fact that the character image has contrastive changes in the image density, while the gray-scale image has gradual changes in the image density. That is, the absolute value of the quadratic differential value S is large in the character image area, while the absolute value is small in the gray-scale image area. Therefore, the discrimination between the character image area and gray-scale image area can be achieved by judging on the magnitude of the quadratic differential value S.
However, since the discrimination between the character image area and gray-scale image area is based on only the quadratic differential value S in the aforesaid related art, the states of respective peripheral pixels and other factors are ignored. Therefore, there exists a problem that the image area is not necessarily properly determined.
More specifically, the image to be read by the facsimile machine includes a dotted image adapted to a printed photograph as well as the character image and gray-scale image. The dotted image is an image comprising dots orderly arranged in which the shading (or image density) of an original image is expressed by way of variation in dot size and dot density. In accordance with the aforesaid related art, pixels judged to belong to the gray-scale image area include pixels constituting a dotted image such as a printed photograph as well those constituting a photographic image or the like which is originally a gray-scale image.
In the facsimile machine, the gray-scale image is subjected to a half-tone image processing for expressing the gradational image density by way of variation in the dot density for image reproduction. However, if this half-tone image processing is applied to the dotted image which has a shading originally expressed by way of variation in the dot density, moire may be generated. Therefore, the aforesaid related art which is incapable of detecting constituent pixels in discrimination of the dotted image from the gray-scale image cannot necessarily perform a proper image processing.